The present invention relates to lamps to stimulate the growth of plants. Generally, it is desirable that the lamp stimulate the plant to approximately balanced plant growth rather than for example relatively long and thin plants which can be produced by improper blue to red ratios, high growth rate (generally measured by the rate of increase of dry weight of the plant) and efficiency, (typically measured by the amount of increase in dry weight per watt of input). It has been realized that while balanced plant growth can be obtained by a lamp whose spectral distribution diplicates sunlight, that such a lamp would be very inefficient as light would be supplied which would not be used and thus would be wasted energy. The green light for example is generally reflected rather than absorbed. Thus a plant growth lamp should provide the proper ratios of energy in the portions of the spectrum which effect plant growth but should not waste energy in portions of the spectrum which do not effect plant growth.
One type of plant growth illumination system which is commonly used is a combination of Cool White fluorescent lamps and incandescent lamps. This combination has been found to be quite productive for plant growth, but has disadvantages due to the use of the incandescent lamps. The incandescent lamps are shorter lived and expend a large amount of their energy in unused infrared radiation.
Illumination systems using only fluorescent lamps have also been used. One such growth lamp is described in U.S. Pat. No. 3,287,586 issued to E. D. Bickford on Nov. 22, 1966. This patent teaches that wide ranges of ratios of energy in the spectral regions of red to far red are acceptable. It also teaches relatively specific ratios in the spectral regions and the use of lead-intensified calcium tungstate blended with tin activated strontium orthophosphate.
Another fluorescent plant growth system is taught in British Pat. No. 1,231,633 issued May 12, 1971 to E. D. Bickford. This patent also teaches that a wide range in the ratio of red to far red is acceptable. This patent teaches a blend of tin activated strontium calcium magnesium orthophosphate with manganese activated magnesium fluorogermanate, with the fluorescent radiation in the blue and violet being negligible, the blue and violet radiation from the lamp deriving substantially entirely from the mercury discharge.